The present invention relates to cellular radio communication systems in which over-the-air repeaters are used to improve signal coverage. More specifically, the present invention relates to the cancellation of interference arising from unwanted coupling between the repeater antennas.
In a cellular system, connections to a public switched telephone network (PSTN) are provided to mobile and handheld telephone subscribers by means of radio transmissions to and from a base station and the subscriber units. In some subscriber locations within the base station""s coverage area (also known as a cell), the radio signals may be severely attenuated by natural or manmade obstructions, thus rendering the transmissions too weak to be used. The areas in which the transmissions are too weak may be in a xe2x80x9cshadow zonexe2x80x9d caused by a hill or building in the path of the transmissions, as illustrated in FIG. 1. These areas may also be inside a building or underground, such as in a shopping mall or subway station.
It is well known in the practice of cellular engineering that the signals to and from a shadow zone can be xe2x80x9cboostedxe2x80x9d using a device known as a xe2x80x9crepeater.xe2x80x9d The basic configuration of an over-the-air repeater is shown in FIG. 2. In principle, forward link signals from the base station to subscriber units are received by one repeater antenna 200 (the xe2x80x9cdonorxe2x80x9d antenna), are amplified, are retransmitted by a second repeater antenna 210 (the xe2x80x9cservicexe2x80x9d or xe2x80x9creradiationxe2x80x9d antenna), and are received only at the subscriber units. Similarly, in principle, reverse link signals from the subscriber units to the base station (whose frequencies are different from those of the forward link signals) are received by the repeater""s service antenna, are amplified, are retransmitted by the repeater""s donor antenna, and are received only at the base station. The use of a repeater to improve cellular coverage in a shadow zone within the coverage area of a base station is illustrated in FIG. 3. A repeater may also be employed to extend the coverage area of a base station as illustrated in FIG. 4. The signals that are retransmitted by the repeater into the shadow zone or extended coverage area are at the same frequencies as those received by the repeater from the base station.
In practice, in both the forward and reverse signal paths there is a finite amount of transmitted energy that is fed back (coupled) to the respective receiving antennas, due to reflections or to imperfections in the antenna radiation patterns, thereby producing interference at the respective antennas. The amount of such coupled interference is highly dependent on the antennas"" relative positions and orientations, as well as the general propagation environment near the repeater. The isolation between the repeater antennas is defined as the measure of the retransmission output power to the coupled input power, usually expressed in decibel (dB) units. Since there is feedback (coupled interference) in the repeater system, there is the possibility of oscillation under certain conditions. To prevent oscillation, the amount of repeater gain must be no more than the isolation between the antennas. In a repeater system where the isolation is much greater than the repeater gain, the possibility of oscillation is very much diminished. One way to accomplish this objective is to reduce the amount of the coupled interference.
The present invention is intended to reduce the coupled interference signals that appear at the repeater inputs. For a bidirectional repeater, as shown in FIG. 5, two cancellation systems are requiredxe2x80x94one for each direction.
With reference to interference cancellation using external samples, U.S. Pat. Nos. 5,125,108 and 5,584,065, disclose methods are removing interfering signals that are present along with the desired communications signal at the antenna of a communications receiver, assuming that a sample of the interfering signal is available at a separate, auxiliary antenna. In these references, adaptive techniques are employed to adjust the amplitude and phase of the sample of the interfering signal so that, when it is combined with the output of the communication system""s receiving antenna, the interfering signal is cancelled. The means for adjusting the amplitude and phase of the sample of the interfering signal in the cited patents is a circuit known as a vector modulator, which is described in a later section of this disclosure.
With regards to the cancellation of coupled interference in repeaters using baseband regeneration U.S. Pat. No. 4,475,243 describes an apparatus is described for minimizing the xe2x80x9cspilloverxe2x80x9d signal from the transmitter to the receiver in a repeater. In this reference, the received signal is translated to baseband (i.e., the carrier modulation is removed) for amplification (regeneration), then translated back up to the same carrier frequency (i.e., remodulated by a carrier) for retransmission. The system described in this cited patent applies to a single communication signal with narrowband analog voice modulation, and it utilizes an xe2x80x9cinjection signalxe2x80x9d based on sampling the regenerated communication signal in conjunction with mixing and correlation techniques to isolate the spillover component of the input signal so that it can be removed at an intermediate frequency (IF) stage of the receiver.
Furthermore, in U.S. Pat. Nos. 4,701,935 and 4,789,993, a digital microwave radio repeater is described in which the desired digital signal is a single signal and is regenerated (amplified) at baseband. In these references, the transmitter-to-receiver coupled interference component that appears at baseband is canceled by subtracting an estimated baseband interference signal. The estimated baseband interference signal is produced by means of an equalization technique implemented by transversal filters whose characteristics are adaptively determined.
With regards to the cancellation of coupled interference in repeaters not using baseband regeneration, U.S. Pat. No. 4,383,331 adds a repeater a xe2x80x9ctagxe2x80x9d to the modulation of a desired analog voice signal prior to its retransmission. The unwanted reception of the repeater output signal at its input, as coupled or otherwise repeated interference, is detected at the repeater as one or more versions of the desired signal at the repeater input that contain the tag in their modulation. In this reference, the strengths of the interference components at the repeater input are separately measured and this information allows the repeater to subtract out the interference.
Furthermore, in U.S. Pat. No. 5,835,848, multiple wideband signals, as in a code-division multiple access (CDMA) wireless system, are received as desired signals at a repeater""s input antenna, along with coupled interference. In this reference, the repeater circuitry contains a means for canceling the coupled interference by subtracting a delayed and amplitude- and phase-adjusted version of the desired signal at the repeater output antenna. The coupling delay is assumed to be known and the degree of phase and amplitude adjustment that is necessary to perform the cancellation is determined by a calibration procedure. The calibration procedure consists of the steps of:
a. waiting for a period of time when there is no repeater input signal;
b. transmitting a test (pilot) signal in the absence of any desired signal;
c. measuring the characteristics of the portion of the test signal that is coupled from the repeater""s output to its input; and
d. if the desired signal is continuous, interrupting the desired signal briefly at some periodic interval in order to transmit and measure the amount of coupled test signal. In these references, the cancellation signal is a delayed and adjusted version of the repeater output signal, which is the source of the coupled interference, where the amount of interference is estimated by measuring a xe2x80x9ctaggedxe2x80x9d input interference component (as in U.S. Pat. No. 4,383,331) or a test signal-only transmission (as in U.S. Pat. No. 5,835,848).
The object of the present invention is to provide a method and a circuit for canceling interference that is coupled from a wideband repeater output antenna to its input antenna. In particular, with regards to interference cancellation using external samples, the present invention applies to the situation in which the interfering signal that is present at the antenna simultaneously with the desired communications signal is a delayed and attenuated version of the communications signal itself, and a sample of this interfering signal is not available at an auxiliary antenna. Moreover, the interfering signal in the application for which the present invention pertains is practically indistinguishable from the desired signal except for a time delay. Therefore, the present invention provides for the injection of pilot signals at the repeater output antenna as a form of known, intentional interference that is easily distinguished and removed, and that facilitates the cancellation of the interfering signal which arrives at the repeater input antenna by the same path as the pilot signals.
With regards to the cancellation of coupled interference in repeaters using baseband regeneration, the present invention applies to a situation in which baseband regeneration is not possible because the desired signal is in general a wideband combination of multiple cellular signals that cannot be isolated from each other for demodulation and remodulation at the repeater.
Further, the present invention does not require any mixing, as in U.S. Pat. No. 4,475,243 to generate intermediate frequencies in the system; instead, the present invention operates at the original radio frequency (RF) of signal being repeated.
The present invention is also a technique in which one or more modulated pilot signals are used to control cancellation operations at different portions of a wideband spectrum that may contain multiple cellular carrier signals.
Lastly, present invention, the transmitter-to-receiver interference is cancelled through the technique of measuring the phase and amplitude of the pilot signals in order to control cancellation circuitry operating directly at the input and output radio frequencies (RF).
Furthermore, with regards to the cancellation of coupled interference in repeaters not using baseband regeneration, according to the present invention the desired signal is not modulated or interrupted by the technique of the present invention; instead, one or more continuous pilot signals are added to the repeater output signal to provide for a measurement of the coupled interference, simultaneously with the normal processing of the repeater input signal. Also, the present invention uses more than one pilot signal, at different frequencies, to enhance the effectiveness of the CICS over a wide bandwidth.